There are many solutions to this, depending on what's the purpose of the tool. Think about how the actual admins would like to report the absence; what info you'd like to keep for each report; what info will be relevant to the actual users (or what will be displayed somewhere else in the app).
Here's some options:
Depending on how the rest of the form looks like; are there other fields related to the absence? I'm guessing you might have something like a reason for absence, in which case you could have a list of predefined reasons and for each one you'd know if you need 1 or 2 date fields.
Or, again, depending on how the rest of the form looks like, you could have two actions: report planned absence and report indefinite absence.
Or, you could make the app smart and handle the dates regardless if they entered only one date or both. Enter both dates, it means the absence is planned; enter only one it maybe means the absence is indefinite.
When the absence is indefinite, you'd want to show the admin some confirmation on submit, letting him know that the absence will be reported as indefinite and also asking him if he wants to submit it like that or cancel and add second date.